To understand the mechanisms by which the mouse epidermal growth factor receptor (EGF-R) produces morphological changes during mammalian development, a null allele (Egfr-m1Cwr) at the locus encoding the receptor was created. Surprisingly, three dramatically different phenotypes were observed depending on genetic background. Inner cell masses of Egfr-m1Cwr homozygotes on a CF-1 genetic background died at implantation. When left co-isogenic on a 129/Sv genetic background, Egfr-m1Cwr homozygotes died at mid-gestation from apparent placental failure. In contrast, the same mutant allele resulted in a peri-natal lethality when bred to homozygosity on a CD- l or C57BL/6 background. Reciprocal uterine transfers between strains showed that the resulting phenotypes were specific to the genetic background of the embryo and were not influenced by the genetic background of the host uterus. These findings have major implications regarding previous work on Egfr since virtually all experimental observations have been made within the context of undefined genetic backgrounds. To understand how these genes are able to compensate for the absence of EGF- R, a genetic analysis is proposed to determine the number and map positions of those genes involved. Appropriate statistical methods are described for the unique situation of exclusion mapping for a modifier(s) allowing for survival beyond a distinct time point. The ultimate goal is cloning of the genes responsible for the effect. A second step toward understanding the biological nature of the variable phenotypes is to determine whether the peri-implantation and mid-gestation Egfr-m1Cwr homozygous phenotypes are actually manifestations of a trophoblast defect that is altered by strain-specific modifiers. This will be accomplished by constructing aggregation chimeras to determine whether EGF-R is essential for differentiation of trophoblast and/or inner cell mass. Continuing the developmental assessment of EGF-R function, experiments are also outlined for determining the requirement for EGF-R during growth and maturation of oocytes as well as during the early pre-implantation stages. These time points could not be addressed by the Egfr-m1Cwr null allele because of the presence of an oocyte-derived pool of EGF-R. Finally, an important aspect for understanding developmental regulation of EGF-R activity is the biological relevance of a truncated form of the receptor that is secreted but lacks kinase activity. The hypothesis is that the secreted receptor modulates full-length receptor activity by competing for ligand. To test this possibility, the secreted receptor will be deleted or overexpressed using gene targeting and transgenic technology.